Add architecture documentation and diagram (#338)

- Adds `docs/architecture.rst`: comprehensive architecture overview covering all 10 modules, end-to-end treatment workflow, class hierarchies, dependency graph, and design patterns (~700 lines)
- Adds `docs/openlifu_architecture.svg`: technical architecture diagram showing module structure, line counts, data flow, and the 10-step treatment pipeline
- Updates `docs/index.rst` toctree to include the new architecture page

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: p1p3dream